Commercial television receivers and set-top boxes that support various digital television standards receive audio and video programming via a wide-band radio-frequency (RF) signal comprising multiple channels. The wide-band RF signal can be received over-the-air (e.g., from a terrestrial broadcast), through a cable (e.g., a coaxial cable), or from a satellite.
These wide-band signals include channels that are distributed over a wide spectrum. For example, the channels may be distributed in the spectrum between 42 MHz and 1002 MHz. A tuner within the receiver selects a single channel from the available channels. The tuner is agile so that a user can select different channels. The objective is to filter out the energy from the undesired channels, leaving only the energy from the desired channel.
The power of the received wide-band signal may be low or high. When the power is low, the receiver should be able to boost the signal significantly without adding substantial noise. When the power is high, the receiver should not saturate.
To boost the power of the received signal, the receiver may include a wide-band low-noise amplifier (LNA) that amplifies the received wide-band signal before the tuner selects one of the channels. The LNA boosts the inbound signal level prior to the frequency conversion process, which prevents mixer noise from dominating the overall receiver front-end performance. Because the received signal may be weak, it is desirable for the LNA to have a low noise figure so that the effect on the input signal of any noise generated by the LNA is low.
Unwanted signals can couple into the desired channel because of harmonic, inter-modulation, and other nonlinear effects. For example, because of the wide-band nature of the received frequency band, there can be an interfering channel with a frequency that is half or a third of the desired channel's frequency. Through second-order or third-order distortion, this interfering channel can cause interference to the desired channel, thereby corrupting the picture quality. Furthermore, a single large interferer can saturate the LNA. Nonlinearities in the LNA may also cause interferers to generate inter-modulation components in the desired channel. These inter-modulation components add noise to the desired channel, thus decreasing the sensitivity of the receiver.
Therefore, it is desirable for the LNA to provide, over a wide range of frequencies, a sufficiently large gain, adequate linearity, a low noise figure, and source impedance matching, while allowing some variable gain to enable the LNA to handle interference.
A balanced common-gate wide-band LNA was described by W. Zhuo et al. in the paper entitled “Using Capacitive Cross-Coupling Technique in RF Low Noise Amplifiers and Down-Conversion Mixer Design,” published in Proceedings of the 26th European Solid-State Circuits Conference, held in Stockholm, Sweden from Sep. 19-21, 2000, which is hereby incorporated by reference for all purposes. An LNA in accordance with the disclosures of Zhuo is illustrated in FIG. 1. As shown in FIG. 1, the single-ended input signal is converted to differential signals by a passive off-chip balun. The differential signals flow into the sources of the common-gate-connected transistors labeled M1A and M2A. They then flow out of the drains of the two transistors. The source of transistor M1A is AC-coupled to the gate of transistor M2A by capacitor C21, and the source of transistor M2A is AC-coupled to the gate of transistor M1A by capacitor C11.
The symmetrical structure of the LNA of FIG. 1 reduces second-order nonlinearities. The use of the capacitor cross-coupling technique also benefits gain boosting and noise cancelling. Therefore, the noise figure of the LNA of FIG. 1 is better than the noise figure of a basic common-gate LNA. But the LNA of FIG. 1 has two apparent drawbacks. First, the off-chip passive balun introduces a loss and increases the cost of the LNA. Second, to meet input impedance matching requirements, the transconductances of transistor M1A and M2A are relatively high, resulting high power consumption.
Another wide-band LNA with thermal noise canceling was described by Bruccoleri et al. in the paper “Wide-band CMOS Low-Noise Amplifier Exploiting Thermal Noise Canceling,” published in the February 2004 issue of the IEEE Journal of Solid-State Circuits, volume 39, no. 2, pp. 275-282, which is hereby incorporated by reference for all purposes. An LNA in accordance with the disclosures of Bruccoleri, referred to herein as the common-source common-gate (CG-CS) LNA, is illustrated in FIG. 2. To accomplish single-ended-to-differential conversion, the CG-CS LNA positively amplifies the single-ended input signal by the common-gate-connected transistor M1B and negatively amplifies the single-ended input signal by common-source transistor M2B. The CG-CS LNA can completely cancel transistor M1B's channel noise current under the conditions of input impedance matching and balanced output. Thus, the noise figure of the CG-CS LNA is relatively low. The nonlinear production of transistor M1B can also be cancelled by the same mechanism, but the second-order and third-order nonlinear effects of the common source connected transistor M2B limit the dynamic range of the CG-CS LNA.